1. Field of the Invention
The present invention relates to a photographic lens configured to reduce variation on an image plane that may occur during focusing.
2. Description of the Related Art
It is desired by the market that a photographic lens used in a single-lens reflex camera has a relatively long back focus to secure a space for providing a mirror of the camera.
In a so-called retrofocus type photographic lens, in which a front lens unit having a negative refractive power is disposed on an object side of a stop and a rear lens unit having a positive refractive power is disposed on an image side of the stop, the position of a rear principal point of the entire optical system can be located at a position relatively close towards the image side. Accordingly, by using a retrofocus type photographic lens, a relatively long back focus can be achieved.
A photographic lens discussed in each of U.S. Pat. Nos. 6,621,645 and 6,697,202 executes focusing on a closest distance object by moving a rear lens unit having a positive refractive power towards the object side. In this case, however, a large amount of variation on the image plane may occur because in a front lens unit having a negative refractive power, the height of a principal ray of an off-axis light flux H may greatly vary during focusing.
In order to reduce variation on the image plane that may occur during focusing on a closest distance object, a conventional photographic lens executes focusing by moving a plurality of lens units. For example, a photographic lens discussed in Japanese Patent Application Laid-Open No. 62-291613 executes focusing on a closest distance object by moving a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power towards the object side.
However, because the front lens unit having a negative refractive power includes a plurality of negative lenses of a great lens diameter, the weight of the front lens unit becomes great. Therefore, characteristics of response of an automatic focusing function may degrade.
A photographic lens discussed in U.S. Pat. No. 5,805,349 includes, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit (a front lens unit of a second lens unit) having a positive refractive power, a stop, and a third lens unit (a rear lens unit of the second lens unit) having a positive refractive power. In addition, in the photographic lens discussed in U.S. Pat. No. 5,805,349, during focusing on a closest distance object, the first lens unit stays stationary while the second lens unit and the third lens unit are moved towards the object side.
However, in the photographic lens discussed in U.S. Pat. No. 5,805,349, because an image forming magnification of the second lens unit is smaller than the equal magnification, focusing on a closest distance object is executed by moving the second lens unit towards the object side.
Therefore, the height of passage of a principal ray of an off-axial light flux may greatly vary. Accordingly, in the above-described conventional method, it is difficult to reduce variation on the image plane.